The present invention concerns a rotary positive displacement machine for a compressible working fluid with intermesh between two cooperating members. The machine is primarily intended for use as a compressor or a vacuum pump but may also be used as an expander or a metering device.
Up to now such machines have generally been of two different types.
The first of those types is the screw rotor machine comprising two externally intermeshing rotors of different profiles enclosed in a casing and rotatable in opposite directions around spaced parallel axes. An example of such a machine is shown in U.S. Pat. No. 3,423,017. In this type of machine one groove in each rotor communicate with each other and form a closed chevron-shaped chamber covered by confronting portions of the barrel wall and of the high pressure end wall. The volume of this closed chamber varies as the rotors rotate. As the rotor land tips normally do not meet on the intersection line between the two barrel sections of the casing a blow hole is formed which means a leakage opening from the chamber to the consecutive chevron-shaped chamber. Furthermore there is always a certain clearance between the end surfaces of the rotors and the high pressure end wall of the casing which results in a certain leakage from the high pressure phase of the machine directly to its low pressure phase, as a portion of the high pressure end wall always cooperates with rotor grooves communicating with the low pressure channel of the machine.
The second of those types is the so called Scroll compressor comprising two members each having a spiral element extending axially from a flat disk. Examples of such a machine are shown in U.S. Pat. Nos. 4,259,043 and 4,395,205. A first member of the machine is held stationary whereas the second member is kept against rotation while its centre is orbiting around the centre of the first member. The spiral elements are dimensioned such that they cooperate alternatingly on one side and the other thereof to form closed pockets therebetween. Those pockets are further sealed off from each other by axially movable sealing strips provided in grooves in the tops of the spiral elements for cooperation with the flat surface of the other disk which unavoidably results in certain leakage openings along the sealing strips partly between the ungrooved top and the disk, partly between the strip and the walls of the groove. The machine further requires means for accurate guiding of the second movable member, thrust bearings to keep the clearance between the members on a small positive value, and means for transforming the rotation of the driving shaft into an oscillating movement of the movable member.
It has further been suggested, as disclosed in U.S. Pat. No. 2,733,854, to make a compressor composed of two sealingly cooperating conical members with coinciding apices intermeshing internally by a hypocyclic motion around the common apex point with a speed ratio of 2 to 1. The machine patented and disclosed is restricted to a type where at least the inner member, shaped as a rod twisted into a conical coil, is manufactured by casting and dimensioned such that it will keep its shape independent of any shrinkage. This means that all dimensions thereof must be direct proportional to the distance from its apex. The axial lead of the coil is consequently proportional to the distance from the apex, whereas the ptich angle is constant. However, this design unavoidably results in a fundamental disadvantage due to the fact that dependent upon the varying axial lead it is impossible to insert the inner member into the outer member without deformation of at least one of the members. For this reason the outer member in the suggested machine is and has to be manufactured from a resilient material. This fact means that when the machine is in function and the pressure inside thereof increases a certain deformation of the resilient member is unavoidable resulting on one side in an increased leakage between the two members, and on the other side in an increased contact pressure between the two members in position opposite to the maximum deformation resulting in increased friction therebetween. In other words the volumetric efficiency of the machine decreases simultaneously as the mechanical losses therein increases resulting in a spoiling of the overall efficiency to such a degree that the machine cannot be used in practice.
A similar type of machine is shown in DE-OS No. 2 736 590. This machine, intended for use as a pump for high-viscous liquids, is still more specialized in that the inner member is shaped as a conical coil wound from a circular rod with constant cross section where the centre of the circle in any axial plane is disposed on the pitch circle of the member. However, also this machine is provided with an outer member manufactured from resilient material and consequently has the same disadvantages as those of the machine disclosed in U.S. Pat. No. 2,733,854.